docs: def perimeter(self):
          """
          The total perimeter of the source segment, approximated lines
          through the centers of the border pixels using a 4-connectivity.
  
          If any masked pixels make holes within the source segment, then
          the perimeter around the inner hole (e.g. an annulus) will also
          contribute to the total perimeter.
          """
  
          if self._is_completely_masked:
              return np.nan * u.pix  # unit for table
          else:
              from skimage.measure import perimeter
              return perimeter(~self._total_mask, neighbourhood=4) * u.pix